A monoclonal antibody against cytochrome P-450c inhibited the conversion of R-propranolol and S-propranolol to 4-Hydroxypropranolol (40HPr), 5-hydroxypropranolol (50HPr), and desisopropylpropranolol (Desipripr). The ratio of the products formed from R or S propranolol by the antibody sensitive portion of the enzyme activities in liver, kidney and lung microsomes from BNF treated rats were remarkably similar. There was also very little intraspecies differences in the metabolism of R or S propranolol by the antibody sensitive portion of the enzyme activities in different tissues of mice, or guinea pigs treated with BNF. However, the ratios of desiprPr/50HPr from R enantiomer in all tissues and species were always higher than those values from S-propranolol. This finding suggests that N-isopropyl side chain group of S propranolol molecule is less likely to be metabolized by the forms of cytochrome P-450 blocked by the antibody than is R-propranolol.